The simultaneous collection of Raman spectral information from many points along a line is called Raman line imaging, and the technique has proven useful in the analysis of compositional variation in a sample as a function of position. Raman mapping, which involves the sequential measurement of Raman spectra from many points on a line, is similar to Raman line imaging, but with important distinctions. First, data acquisition times are much longer. Second, many samples change with time, so the Raman data from all points on the line need to be acquired at the same time for meaningful conclusions to be drawn.
Raman line imaging has traditionally been done by illuminating a line on the sample with a laser, and imaging that line onto the entrance slit of a spectrograph. A two-dimensional detector attached to the spectrograph records spectra of each point along the entrance slit height. The number of Raman spectra in the Raman line image is determined by the point spread function of the instrument and the number of detector elements along the illuminated slit height image at the plane of the detector.
Three methods of sample illumination have been used for Raman line imaging. One method simply uses a 90.degree. collection geometry with a transparent sample. This uses the laser photons very efficiently, but only works for transparent samples, and may be very sensitive to sample alignment and morphology. A second technique uses a cylindrical lens for line illumination. This method addresses the limitations of the first method, but the laser intensity along the line is not constant, due to the Gaussian profile of the laser beam. Raman line images collected with this illumination method must also be corrected for the non-uniform laser intensity along the line. A third illumination method overcomes laser intensity non-uniformity by rapidly scanning a laser spot to illuminate the line-shaped region.
All of the Raman line imaging approaches reported so far, however, rely on direct imaging of the illuminated sample region onto the entrance slit of the spectrograph. Fiber optic coupling of the sample emission to the spectrograph has, however, proven advantageous over direct coupling for many types of single-point measurements. The benefits of fiber optic coupling over direct coupling include ease of use, ruggedness, and operation in hostile environments.